STOP + RESTORE now works (bad news: works even without STOP)

c64/kernal/interrupts/nmi_handler.s

@@ -1,7 +1,18 @@
 
 nmi_handler:
 
-	;; XXX provide real implementation
+	sei ;; don't allow IRQ to interfere, see https://www.c64-wiki.com/wiki/Interrupt
 
+	;; Call interrupt routine (only if initialised)
+	pha
+	lda NMINV
+	ora NMINV+1
+	bne +
+	pla
 	jmp (NMINV)
+*
+	;; Vector not initialized - call default interrupt routine
+	pla
+	jmp default_nmi_handler
+